One thing that i do, with autofocus, is focus on an object very similar to the fish (i.e. a rock/plant) and then hold the shutter button half way, until a fish gets in the perfect position.
As well, focus uses contrast in order to gauge if it has focus or not. Using something that has a line of light/dark in it aids greatly.
Macro function gives the camera more preference to focus very closely. It will try the closest settings first.
I'm not into the SLR scene, i just have a nikon coolpix 5900, but i am insanely happy with its macro. I just wish it could take in more light. My photos without flash come either blurry (low ISO setting) or underexposed (high ISO numbers) with a larger sensor of an SLR, it can allow more light in during the same shutter speed. Perhaps my next camera will be an SLR
My dad has a Nikon D70, i just have to steal that one of these days